I have plastic plumbing throughout my house.
Does that mean you have CPVC piping?
I saw some big chunks float away towards the kitchen sink.
How could you do that? How can you see inside a pipe?
A build up of anything around a joint indicates a leak at that joint.
Water pressure and volume of hot and cold at each faucet should be equal or nearly equal. A significant difference indicates a restriction of some kind.
What kind of faucets do you have, single handle or two knobs?
Suggestion, remove hot stem or cartridge at kitchen faucet. Hold a cup over opening and turn of water briefly to flush line.
Alternative suggestion, if you have flexible supply lines to faucet, switch hot to cold and cold to hot. If the reduced pressure and volume moves to cold that would indicate a restriction in the hot side of the faucet.
I have drained the sediment from the tank
Does that mean you drained the tank or does it mean you flushed the water heater? There is a difference.
How old is water heater?